Can I travel in countries where my pass is not valid?
No, your pass only allows you train travel through the 3, 4 or 5 bordering countries you select. If you want to travel in a country that is not covered, you'll need to buy a separate ticket that covers that part of the journey. You can do so at the local train station of that country. Check the map to find out which countries are considered bordering countries.
If you want to travel in a country not covered by your pass, you need to buy an additional ticket for that section of the trip. Buy this ticket locally at a train station. Buying tickets on the train is often more expensive and you risk getting a fine.
You plan to travel from Munich (Germany) to Venice (Italy) with a Select Pass - 3 countries, valid in Germany, Italy and Switzerland. The fastest route is through Austria, which is not included in your pass. There are 3 possible solutions:
Order a different Eurail pass, one that is also valid in Austria.
Buy an additional ticket in advance at a German station, just for the part of your journey in Austria.
Research an alternative route, to stay within the countries where your pass is valid. You could avoid Austria by traveling through Switzerland, though this would take you longer time.